This invention relates to automatic transfer apparatus that is particularly useful in situations in which a workload must be handled in a clean environment, such as in the semiconductor industry. In the semiconductor industry, carriers or cassettes of semiconductor wafers are transferred to many different work stations for various processing steps. Extreme cleanliness is required during these steps and in processing the wafers in order to improve the yield of an acceptable product. A multitude of extremely small and complex circuits are formed on the wafers. It only takes a small particle of dust or other contaminant to either short circuit the imprinted circuitry or to block processing chemicals from reaching every portion of the circuitry imprinted on the wafer. Because of the cost of semiconductor devices and the cost of the systems and equipment in which such devices are used, it is extremely important that the necessary quality levels be attained.
Currently, the various processing steps are typically conducted in "clean rooms" wherein the environment is controlled to keep contaminates below a certain predetermined level. One difficulty that arises is the method of keeping the wafers clean when being moved from one processing area to another. Because of the many steps involved, it is usually impractical or impossible to arrange them all in a continuous sequence or in a single room wherein the entire operation is conveniently maintained in a single clean environment. Instead, in moving from one area to another, it is usually necessary to cross areas not having an environment which is controlled to the necessary levels. Typically, the work in process is manually transported to the next work station and the wafers are covered in some manner, such as a box, that will hopefully maintain the necessary cleanliness. This approach is, in general, not very satisfactory and results in a lower yield of acceptable product. Manual handling often also results in damage to the fragile wafers.
Early attempts to automate the transport process by the use of conveyor belts and platens or by air or by vibratory tracks did not solve either the contamination or the abrasion problem. Conveyor systems subject the wafers to numerous vibratory loads. The air transport systems increase the risk of contamination through the very air used to transport the wafers. Vibratory tracks increase the abrasion of the wafers and platen conveyor systems subject the wafers to numerous vibrations as they are rolled over each platen, thereby increasing the chances of abrasion and increasing the shock loads tending to fracture the wafers.
U.S. Pat. No. 4,540,326 discloses an enclosed horizontal conveyor system that is capable of properly moving a load of wafers from one processing area to another in a manner that does not damage the wafers and maintains the necessary cleanliness. This system is disclosed in the drawings and description of the present application. As a supplement to that system, a need exists for locating the horizontal conveyor system and for providing automated means for inserting and withdrawing loads from the horizontal conveyor.